Over 170 Separex™ membrane systems have been installed in the world for gas separation applications such as for the removal of acid gases from natural gas, in enhanced oil recovery, and hydrogen purification. Two new Separex™ membranes (Flux+ and Select) have been commercialized recently by Honeywell UOP, Des Plaines, Ill. for carbon dioxide removal from natural gas. These Separex™ spiral wound membrane systems currently hold the membrane market leadership for natural gas upgrading. These membranes, however, do not have outstanding performance for olefin/paraffin separations. Development of new stable and very high selectivity membranes is critical for the future success of membranes for olefin/paraffin separation applications such as propylene/propane and ethylene/ethane separations.
Light olefins, such as propylene and ethylene, are produced as co-products from a variety of feedstocks in a number of different processes in the chemical, petrochemical, and petroleum refining industries. Various petrochemical streams contain olefins and other saturated hydrocarbons. Typically, these streams are from stream cracking units (ethylene production), catalytic cracking units (motor gasoline production), or the dehydrogenation of paraffins.
Currently, the separation of olefin and paraffin components is performed by cryogenic distillation, which is expensive and energy intensive due to the low relative volatilities of the components. Large capital expense and energy costs have created incentives for extensive research in this area of separations, and low energy-intensive membrane separations have been considered as an attractive alternative.
In principle, membrane-based technologies have the advantages of both low capital cost and high-energy efficiency compared to conventional separation methods for olefin/paraffin separations, such as propylene/propane and ethylene/ethane separations. Four main types of membranes have been reported for olefin/paraffin separations. These are facilitated transport membranes, polymer membranes, mixed matrix membranes, and inorganic membranes. Facilitated transport membranes, or ion exchange membranes, which sometimes use silver ions as a complexing agent, have very high olefin/paraffin separation selectivity. However, poor chemical stability, due to carrier poisoning or loss, high cost, and low flux, currently limit practical applications of facilitated transport membranes.
Separation of olefins from paraffins via conventional polymer membranes has not been commercially successful due to inadequate selectivities and permeabilities of the polymer membrane materials, as well as due to plasticization issues. Polymers that are more permeable are generally less selective than are less permeable polymers. A general trade-off has existed between permeability and selectivity (the so-called “polymer upper bound limit”) for all kinds of separations, including olefin/paraffin separations. In recent years, substantial research effort has been directed to overcoming the limits imposed by this upper bound. Various polymers and techniques have been used, but without much success in terms of improving the membrane selectivity.
More efforts have been undertaken to develop metal ion incorporated, high olefin/paraffin selectivity facilitated transport membranes. The high selectivity for olefin/paraffin separations is achieved by the incorporation of metal ions such as silver (I) or copper (I) cations into the solid nonporous polymer matrix layer on top of the highly porous membrane support layer (so-called “fixed site carrier facilitated transport membrane”) or directly into the pores of the highly porous support membrane (so-called “supported liquid facilitated transport membrane”) that results in the formation of a reversible metal cation complex with the pi bond of olefins, whereas no interaction occurs between the metal cations and the paraffins. Addition of water, plasticizer, or humidification of the olefin/paraffin feed streams to either the fixed site carrier facilitated transport membranes or the supported liquid facilitated transport membranes is usually required to obtain reasonable olefin permeances and high olefin/paraffin selectivities. The performance of fixed site carrier facilitated transport membranes is much more stable than that of the supported liquid facilitated transport membranes and the fixed site carrier facilitated transport membranes are less sensitive to the loss of metal cation carriers than the supported liquid facilitated transport membranes.
Refinery grade propylene separation involves conventional distillation columns. For customers who want to debottleneck or expand capacity without having to install additional columns, the present invention provides a membrane solution that can process additional capacity required and selectively separate propylene from propane to produce very high purity propylene. The membrane solution can be cost effective and offer flexibility for customers with faster startup than conventional column installations. The payback time (based on lower investment) is also a very attractive solution for processing additional throughput.
It has been found that effective processing of olefin/paraffin streams can be accomplished with membranes. A modular approach can also be of great advantage for skid-built, low installation factor and faster startup than conventional column systems.
For an existing distillation system to produce 200 KMTA (metric tons per year) propylene (refinery grade 93 mol %), the column diameter is expected to be 3.96 meters (13 feet) with 140 trays in one embodiment. To debottleneck this system, a membrane system with 200-1000 elements, preferably 400-700, can be installed to bring in additional 136 KMTA propylene, making a total of 336 KMTA for the customer. In order to achieve the same additional capacity by using the prior art distillation column technology, a new column of 11 feet can be used.
The erected cost for a new column system is estimated to be more than double the cost of a new membrane system. The payback time for the above example is 3-6 months, which is about twice lower compared to the payback from the equivalent column system. Even with compression, the operating costs remain equivalent to the column system in which there would be two columns as compared to the single column plus the membrane system in the present invention.
The feed to the membrane system can be an effluent stream from a propane dehydrogenation (PDH) process, a FCC process or other source of a light olefin stream with various concentration of propylene, ranging from 30-70 mol %. This stream is contacted with a membrane separation unit. The permeate from the membrane system is propylene rich and has a high concentration of propylene, 93-99% or higher and in some embodiments of the invention from 95-98%. The flow rate of the permeate stream is 25-50% of the feed stream, or preferably 30-40%. The retentate stream can be compressed up to 220-250 psig and then sent to a C3 splitter column.
The distillation column is typically operated at 150-250 psig, preferably at 180-230 psig. Lower pressure can be achieved, however, condensing the vapor overhead at the top of the column which may require chilled water, if available. In some embodiments, the feed to the column may pass through a shared drier system to reduce water before contacting with the column. The column condenser temperature is 32.2 to 48.9° C. (90-120° F.), preferably 37.8 to 43.3° C. (100° to 110° F.) (optimized for cooling water condensing). The reboiler temperature is 37.8° to 60° C. (100° to 140° F.), preferably 43.3° to 51.7° C. (110° to 125° F.). For new units, the technology that the tray spacing can be optimized so tray spacing is less than 20 inches, increase efficiency with less overall height (up to 30% height reduction compared to regular trays). The composition of the distillate is 80-98 mol % propylene, or 88-95 mol % propylene in some instances. The recovery of propylene through the column is 90-98%, or preferably 93-96%.
The permeate from the membrane (95-99 mol % C3=) can be mixed with the distillate stream (88-95%) to produce a refinery grade propylene product stream. The propylene product stream may go through a shared drier to remove water.
For an existing Oleflex or PDH system, a portion up to all of the retentate can be compressed and contacted with a secondary membrane system to further produce a second permeate (95-99 mol % propylene) and a second retentate (>75 mol % propane) that can be compressed and recycled to the reactor system to further convert to propylene. In any embodiments, the bottom of the distillation column, which has greater than 90 mol % propane, can be recycled back to the dehydrogenation unit. The above discussion is mainly concerning a membrane unit or system placed in front of an existing column to result in a system that can produce about 93 mol % propylene. It has also been found that a system can be installed with the membrane system placed after the column and then produce even higher levels of propylene in its product stream.